1. Field of the Invention
This invention relates to an apparatus for heating water. More particularly, this invention relates to condensing water heating systems. In one aspect, this invention relates to liquid- or gaseous-fuel fired condensing water heating systems. In one aspect, this invention relates to condensing gas-fired commercial or residential water heating systems which utilize heat and moisture from the flue gases generated by the combustion process to humidify and preheat the combustion air employed in the combustion process.
2. Description of Related Art
Conventional condensing water heaters, which have an extended flue gas-to-water surface area within the storage tank (or the reverse in the case of condensing tankless water heaters), are limited by the heat sink temperature, i.e. the stored or incoming water, and the dew point of the flue gases. Although truly condensing devices, these water heaters typically advertise an “up-to” efficiency rating, acknowledging that, for example, storage-based water heaters may only reduce flue gas temperature to that of some portion of the storage tank. When such a storage-based water heater fires in standby, whereby the thermostat dead band has been surpassed (i.e. the tank is warm or hot), the flue gases may not even condense depending on the thermostat set point. It should be noted that the estimated efficiency is without regard to the system state due to a static heat sink. That is, this is not an “up-to” efficiency due to the flue gases being condensed effectively by ambient conditions rather than the water tank itself.
Currently, there are a number of condensing tankless and storage-based water heaters available on the market for both commercial and residential applications. These units claim “up-to” 99% thermal efficiency; however, this is a state dependent efficiency, and for storage systems, it requires that the stored water temperature be non-uniform, that is, cold near the flue gas exit. The cost of condensing water heaters, both commercial and residential, is dependent upon its primary technology, the glass-lined flue/heat exchanger. The greater the heat exchanger surface area, the greater the efficiency, performance, and cost. This is particularly problematic for condensing versus non-condensing storage-based water heaters, as the heat sink is not as consistently dynamic as tankless water heaters.